Raise a ValueError if no match.

Closes #488

Author: jaraco

importlib_metadata/__init__.py

@@ -155,6 +155,22 @@ class EntryPoint:
     'attr'
     >>> ep.extras
     ['extra1', 'extra2']
+
+    If the value package or module are not valid identifiers, a
+    ValueError is raised on access.
+
+    >>> EntryPoint(name=None, group=None, value='invalid-name').module
+    Traceback (most recent call last):
+    ...
+    ValueError: ('Invalid object reference...invalid-name...
+    >>> EntryPoint(name=None, group=None, value='invalid-name').attr
+    Traceback (most recent call last):
+    ...
+    ValueError: ('Invalid object reference...invalid-name...
+    >>> EntryPoint(name=None, group=None, value='invalid-name').extras
+    Traceback (most recent call last):
+    ...
+    ValueError: ('Invalid object reference...invalid-name...
     """
 
     pattern = re.compile(
@@ -211,7 +227,13 @@ def extras(self) -> list[str]:
     @property
     def _match(self) -> _EntryPointMatch:
         match = self.pattern.match(self.value)
-        assert match is not None
+        if not match:
+            raise ValueError(
+                'Invalid object reference. '
+                'See https://packaging.python.org'
+                '/en/latest/specifications/entry-points/#data-model',
+                self.value,
+            )
         return _EntryPointMatch(**match.groupdict())
 
     def _for(self, dist):